The present invention relates to a lid opening-closing mechanism for moving a lid between an open position and a closed position, and also relates to a storage device provided with the lid opening-closing mechanism.
FIGS. 9(a)–9(b) and 10(a)–10(b) show conventional storage devices. As shown in FIGS. 9(a) and 9(b), in the storage device disclosed in Japanese Patent Publication (Tokkai) No. 2001-336335, a lid 42 rotates relative to an upper opening of a box-like main member 41 to switch between a closed position shown in FIG. 9(a) and an open position shown in FIG. 9(b). A lid opening-closing mechanism includes four pins, i.e. two pairs of pins 44a and 44b, projecting on both sides of the lid 42; guide grooves 46 provided on both sides of the main member for receiving the pin 44a and 44b; torsion springs 43 for urging the lid 42 in two directions; and rotation plates 45 supported on axes 48 projecting on both sides of the main member and having clearance grooves 45a engaging ends of the pins 44a. The guide groove 46 has a branch groove 46a at an end thereof corresponding to a state where the lid 42 is at the closed position shown in FIG. 9(a). One end of the torsion spring 43 is fixed to an engagement part 47 on the main member 41, and the other end is fixed to the pin 44a. 
The above-mentioned lid opening-closing mechanism works as follows. When the lid 42 is at the closed position shown in FIG. 9(a), the pins 44b are located at upper ends of the guide grooves 46, and the pins 44a are located in the branch grooves 46a. Also, the torsion springs 43 urge the lid 42 toward the closed direction. When switched to the open position, the lid 42 rotates around the pins 44b by a predetermined angle toward the open position against the urging forces of the torsion springs 43. After the pins 44a come to the guide grooves 46 from the branch grooves 46a, both pins 44a and 44b move to lower end sides of the guide grooves 46 as shown in FIG. 9(b). The torsion springs 43 urge the lid 42 to move toward the open position when the pins 44a enter the guide grooves 46 from the branch grooves 46a. 
Similar to the device shown in FIGS. 9(a)–9(b), in the storage device shown in FIGS. 10(a)–10(b), a lid 52 rotates relative to an upper opening of a main member 51 to switch between a closed position shown in FIG. 10(a) and an open position indicated by an imaginary line in FIG. 10(b). The storage device shown in FIGS. 10(a)–10(b) has a lid opening-closing mechanism different from that shown in FIGS. 9(a)–9(b). The lid opening-closing mechanism includes pins 54a, 54b provided on both sides of the lid 52; guide grooves 56 provided on both sides of the main member 55 for receiving the pins 54a, 54b; coil springs 53 for urging the lid 52 toward the open position; a locking device (not shown) for holding the lid 52 at the closed position; and damper devices 58 disposed on sides of the main member.
The guide groove 56 includes a branch groove 56a in the same way as described above. Each of the coil springs 53 has one end fixed to an engagement part 57 of the main member 51 and the other end fixed to the pin 54a to provide a pulling force between the engagement part 57 and the pin 54a. The damper device 58 is a rotary type with a rotational axis 58a. The rotational axis 58a is connected to one end of the arm 59 to rotate together. The other end of the arm 59 is connected to the pin 54b. 
The lid opening-closing mechanism works as follows. In the closed position shown in FIG. 10(a), the lid 52 engages the locking device against the urging force of the coil springs 53 urging the lid toward the open position. When the lid 52 is opened, the lid is released from the locking device to switch to the open position. As shown in FIG. 10(b), the pin 54a moves out of the branch groove 56a while pivoting around the pin 54b and moves along the guide grooves 56, so that the coil springs 53 urge the lid 52 to move toward the open position. Specifically, when the pin 54a moves in the branch groove 56a for a distance a shown in FIG. 10(a), the pin 54b does not move. The lid 52 rotates around the pin 54a, and moves at a damped speed along with the arm 59 of the damper device 58. When the pin 54a reaches all the way the end of the guide groove 56, the opening of the main member 51 is completely opened.
In the lid opening-closing mechanism in FIGS. 9(a), 9(b), the torsion springs 43 switch the moving direction of the lid 42 depending on the position thereof. When the lid 42 moves closer to the closed position during the opening-closing movement, the torsion springs 43 urge the lid 42 to move toward the closed position. When the lid 42 comes closer to the open position during the opening-closing movement, the torsion springs 43 urge the lid 42 to move toward the open position. When the lid 42 is at the closed position, the lid 42 is urged toward the closed direction by the torsion springs 43. Therefore, it is possible to eliminate the locking device for engaging the lid 42 at the closed position. However, the lid 42 does not move smoothly and tends to cause an undesired sound when the torsion springs 43 switch the moving direction of the lid. In addition, when the lid 42 is opened from the closed position shown in FIG. 9(a), it is necessary to push the lid 42 toward the open position against the urging forces of the torsion springs 43 to switch the urging direction of the torsion springs 43.
On the other hand, in the lid opening-closing mechanism in FIGS. 10(a), 10(b), when the locking device is released, the lid 52 moves from the closed position to the open position due to the urging forces of the coil springs 53. When the lid 52 is closed, the lid 52 moves from the open position to the closed position against the urging forces of the coil springs 53, in other words, the lid 52 is rotated while storing the urging forces for the coil springs 53 (spring charge). The coil springs 53 have the maximum spring forces at the closed position. Therefore, the locking device is required to have a large engagement force. Also, the lid 52 moves too fast in the early stage when the lid 52 is released and moves toward the open position, thereby causing a large variation in the rotational speed. Although the damper device 58 is provided for reducing the variation in the rotational speed, it requires additional labor to assemble to cause a higher cost and an increased weight.
As described above, the conventional lid opening-closing mechanisms have merits and demerits, and it is difficult to obtain a smooth switching operation.
The present invention has been made in view of these problems, and an object of the invention is to provide an opening-closing mechanism in which a lid automatically moves toward the open position when the lid at the closed position is released from a locking device. In the opening-closing mechanism of the invention, it is possible to reduce a variation in a rotational speed of the lid without using the conventional damper device, thereby improving functionality. It is also possible to reduce the maximum spring force of an urging device, thereby making it easy to simplify the locking device.
Further objects and advantages of the invention will be apparent from the following description of the invention.